For instance, the increased expression of IDO and tryptophan-2,3-dioxygenase (TDO) by tumor cells, tumor-associated macrophages (TAMs) and tumor-associated dendritic cells and fibroblasts, which catalyze the conversion of tryptophan to kynurenine, results in tryptophan depletion and contributes to the dysfunction of TILs. This evidence concerns the gene TDO2 and neoplasm.